..... It just feels like various plans and plot threads that were meant to lead somewhere else were just cut short because ?????......

I'm tempted to ask if they have the same planning team as "The Last Jedi".........

As has been noted it's all a bit odd, and various theories could point to:

1) There never was a plan
2) The was plan and they decided part way in that it didn't work
3) There is a plan and the Mechanic will be back in Series 3 (any evidence of that from anything Chris Jarman has said?)
4) There was a plan but it conflicts with what they now want to do with Series 3 (which raises a question: Has there been any evidence that ITV are going to order a Series 4? - i.e 3 will be the last one and they need a different focus)

At least we don't need to wait too long this time to start to find out.